Portland's Quarries and Its Stone
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Portland's Quarries and its Stone Fig 1. The City of London. A monument to the Portland stone industry and its artisans. Mark Godden. Mine and Quarry Manager, Albion Stone PLC. Introduction. I have written this document as a handout for students visiting Portland in Dorset, who wish to gain a basic understanding of the island’s geology and the modern extractive industry that operates here. I have not aimed to give a full and comprehensive account of Portland’s geology or of the dimension stone quarrying and mining techniques used, but instead I have attempted to provide an accessible introduction to these subjects for the interested reader. I have used traditional quarrymen’s terms where appropriate. These would have been familiar to a great many of my ancestors who earned a hard living hewing stone in Portland’s quarries and I will be happy if using them here helps to keep these colourful words alive. Geology. Towards the end of the Jurassic Period, around 145 Ma (million years ago), because of continental drift, the land that makes up much of what is Southern England was located far south of its present global position, at about 38o north of the Equator. What is now Portland was positioned at a palaeolatitude similar to that of modern Florida or Israel, where the “Mediterranean” type climate was likely characterized by warm wet winters and hot dry summers. 1 Portland Stone formed in a specific type of marine environment, known as a carbonate ramp, on the floor of a shallow, warm, sub-tropical sea, not too far from land, as evidenced by carbonaceous driftwood trace fossils, which are not uncommon. As seawater was warmed by the Sun, its capacity to hold dissolved gas was reduced; as a consequence, dissolved carbon dioxide CO2 was released into the atmosphere (as a gas) disrupting the chemical mass balance in the water. In response, discrete calcium cations 2+ - (Ca ) and bicarbonate anions (HCO3 ) within the supersaturated seawater bonded together, forming calcium carbonate (CaCO3) as a solid precipitate, according to the formula: Minute particles of sand or organic detritus, such as shell fragments, lying on or in suspension close to the sea floor, acted as nuclei which gradually became coated with this fine-grained (micritic) calcareous precipitate. Over time more calcium carbonate accumulated (by accretion) around the nuclei in concentric layers, forming small calcareous spheres (< ~1mm diameter). Countless billions of these spherical sediments, called “ooids” or “ooliths”, ultimately became buried and partially cemented together (lithified) by more calcium carbonate, resulting in the oolitic limestone we now call Portland Stone. Fortunately, the degree of cementation in Portland Stone is sufficient to allow it to resist the detrimental effects of the weather, but it is not so well cemented that it can't be readily worked (cut and carved) by masons. This is one of the reasons why Portland Stone is so favoured as a monumental and architectural stone. Using the Folk classification scheme, geologists categorize Portland Stone as mainly porous oosparite. Portland Stone is one of the youngest Jurassic rocks, laid down just before the end of the period. Contemporary (aragonitic) oolitic limestone, that might be considered a modern analogue for Portland Stone is forming today in warm, lime-rich waters, such as those found in the coastal margin of the Persian Gulf and in the Atlantic, on the Bahamas’ Banks. 2 During the Alpine Orogeny (when the Alps were pushed up as Africa tectonically “crashed” into Southern Europe), minor related tectonic “ripples” affected much of the south of England. These forces compressed the already folded rocks of South Dorset, resulting in an east-west trending fold structure called the “Weymouth Anticline”. Portland, connected to the mainland by Chesil Beach (forming a tombolo) is what remains of the Weymouth Anticline's southern limb, where bedding dips at about 1.5 o to the south east (fig. 4.). The Portland Beds which once would have been folded over what is now Weymouth have long since been denuded and only the older middle Jurassic, (generally softer) rocks, originally from the core of the anticline remain, now exposed at the surface. Were it not for the protective barrier of Chesil Beach, Weymouth and its hinterland would probably have long been eroded away and the whole area would now lie beneath sea level. There is a narrow, linear exposure of Portland Stone, (striking roughly east-west) on the high ridge to the north of Weymouth; it crops out (dipping steeply north) on the northern limb of the Weymouth anticline forming part of the escarpment known locally as the Ridgeway. Portland Stone from the Ridgeway tends to be more fractured or jointed (compared to that quarried on Portland) because of the more intense folding that occurred there. Despite geological problems Portland Stone was once quarried from the Ridgeway, particularly at Portesham. The monument to Vice Admiral Sir Thomas Masterman Hardy (1769-1839), erected in 1844 at Blackdown, Portesham, SY 613875, was originally constructed using stone quarried from Portesham Quarry SY610860 although subsequent restoration has been carried out using Portland Stone from Portland. 3 Fig. 2 Titanites. (40cm dia). This specimen came from the Whitbed in Fancy Beach Quarry, Portland. Before the quarrying of Portland Stone can proceed, the unwanted material which overlies it, (the overburden) must be removed. Most of the top of Portland is covered with Purbeck Group limestones (the Lulworth Formation) of quite variable thickness, ranging from about 1m to 15m or more, across the Island (fig. 4.). Where the overburden is deepest; the top 4-5m (Top Rubble) is usually fairly unconsolidated, comprising thin beds of limestone, clay and marl. Stromatolites are common amongst the thin limestone or Slatt Beds which occur at these upper horizons. The largest of these Slatt Beds (the Thick Slatt) is typically 0.5m thick and has been commercially marketed as “Portland Blue“. The Thick Slatt often exhibits some very fine sedimentary structures, including halite (salt) pseudomorphs, bioturbation, ripple marks, rain prints and desiccation cracks, which all point to a seashore type environment, with high rates of sea water evaporation. Similar hyper-saline conditions exist today in the Middle Eastern sabkhas. 4 Fig. 3 Some slabs of Thick Slatt quarried from the south of the Island have been found to contain trydactyl (three-toed) dinosaur footprints. These beds also contain small fossils including gastropods, bivalves, ostracods and very rarely, the occasional disarticulated vertebrate bone. The Thick Slatt's “blue heart” occurs as a result of staining by naturally occurring iron compounds. A distinctive white or gray margin is often visible at the edges of slabs of Thick Slatt, this is caused by vadose water (ground water migrating under the influence of gravity) moving through joints in the bed that has weathered the blue staining from the margins of individual joint-bounded blocks. 5 The lower 5-6m of overburden (the Bottom Rubble) consists of more massive beds of deformed evaporitic limestone (Aish and Soft Burr) with thin basal beds of ancient soil or palaeosol. The “chalky” Aish was traditionally used by Portland's housewives to whiten hearths and doorsteps. The noun “Aish” is probably a corruption of “ash”, a colour term. It is just possible that “Aish” might have a linguistic connection to the noun “ashlar” (squared blocks of building stone) but given the soft and friable nature of Aish, this seems unlikely. Load structures are sometimes found at these horizons, where buried salts (gypsum and halite) that were subjected to high and possibly uneven gravitational loads, were mobilized upwards, forming small “diapiric plumes” which disrupt the overlying beds. The “Soft Burr” found here was traditionally used on Portland to build chimneys and fire places as it has the interesting property of resisting the detrimental effects of heat. It is in the Great Dirt Bed that many well preserved silicified tree trunks are found, some still in life position. The three palaeosols or “dirt beds” at the base of the Lulworth Formation represent ancient soils into which “proto-cypress” conifer trees (Protocupressinoxylon purbeckensis) and cycads once spread their roots, which are still abundantly evident as carbonaceous and limonitic trace fossils. Interestingly, fragments of fossilized charcoal (fusain) are sometimes found in the dirt beds, indicating that the plants that once grew here were affected by occasional forest fires, presumably started by lightning strikes. Cap Beds, comprising the Hard Cap and the underlying Skull Cap, separated by the Lower Dirt Bed, is the collective name given to the basal units of Lulworth Formation and the bottom beds of overburden (fig. 6.). The Hard Cap often contains trace fossils of trees and the frequently encountered horizontal “chaff holes” are what remain of now largely decayed tree branches preserved in something close to life position. Careful examination of the holes does occasionally reveal some traces of petrified wood. The ancient forest floor in which these trees once grew (represented by the Lower Dirt Bed) must have been flooded quickly. Much of the calcium carbonate in the Cap Beds is stromatolitic (algal) in origin, produced by the action of cyanobacteria (blue-green algae), living in backshore lagoons, on and around the newly submerged trees. Ostracod fossils are common within the Hard Cap. The sequence of limestones and palaeosols within the 6 Lulworth Formation tell a story of significant environmental changes, with cyclic flooding and re-emergence of the land surface. Cap Beds are typically 2-3m deep and generally very hard. The noun “Cap” is widely used in mining to describe the rock immediately above a coal or ore seam. On Portland, Cap Beds are those that immediately overlie the economically valuable Freestone Beds of the Freestone Member.